1. Field of the Invention
The invention is directed to a multi-echo magnetic resonance imaging of the type wherein a number of high-frequency pulses are generated including a high-frequency excitation pulse and a number of following high-frequency refocusing pulses, and wherein at least two phase encoding gradient pulses are respectively generated between the high-frequency refocusing pulses.
2. Description of the Prior Art
Imaging technologies on the basis of magnetic resonance signals (MR signals) are used in the medical field for preparing image datasets of a target area. For this purpose, the area to be examined is positioned in a strong homogeneous magnetic field of a diagnostic magnetic resonance apparatus (MR apparatus). A high-frequency excitation pulse, whose frequency is determined by the Larmor frequency, excites the magnetic resonance signals. Subsequent to the excitation and subsequent to a refocusing measure such as a high-frequency refocusing pulse, a magnetic resonance signal is received whose intensity is proportional to the density of the excited particles. In multi-echo imaging methods, further magnetic resonance signals are received by repeated refocusing measures after a onetime excitation. The location encoding of the magnetic resonance signals occurs by means of additional magnetic gradient fields modifying the frequency and the phase of the magnetic resonance signals in a location-dependent manner.
Multi-echo imaging methods, however, are highly sensitive to non-linearities in magnetic resonance apparatuses. The resulting strict requirements with respect to the device technology also relate to the xe2x80x9cfield purityxe2x80x9d of the gradient system which always produces undesired time-dynamic noise fields in addition to the desired useful field. These time-dynamic noise fields cause phase errors in the MR signal with different effects on the image quality. The phase errors lead to interferences, so that the signals are no longer constructively superimposed, but destructively. For example, this results in a position-dependent signal cancellation in the images. Furthermore, so-called ghost images can occur which are caused by the different nature of the interferences of the different echo signals contributing to an image dataset.
A number of causes can be responsible for the origination of the aforementioned time-dynamic noise fields, such as eddy currents of all types and hysteresis effects and residual magnetizations associated therewith.
U.S. Pat. No. 5,729,139 discloses a multi-echo imaging method of the aforementioned type. The method disclosed therein prevents eddy currents and residual magnetizations from impairing the image quality. For this purpose, a modified reset gradient is generated in the phase encoding direction after the magnetic resonance signal has been received. The reset gradient consists of a component and of opposite polarity with a substantially equal gradient-time integrals, and a preceding phase encoding gradient and of a correction component for correcting the influence of eddy currents or residual magnetization caused by the preceding phase encoding gradient. It is disadvantageous hereby that the disturbing component must be known. The additional information about the disturbance variable is measured prior to the beginning of the image generation. The time required for this purpose is lost for the actual determination of the image data.
U.S. Pat. No. 6,043,656 describes a MR imaging system having a gradient compensation system which also compensates residual magnetizations. The gradient compensation system adds reset gradient pulses to the imaging gradient pulses, so that the residual magnetization is kept at a constant value. The reset gradient pulse either is generated after each imaging gradient or is generated only when the imaging gradient pulse has a polarity that is different from the selected residual magnetization. Image artifacts are thus reduced. This method, however, represents a limitation regarding the development of pulse sequences, since a corresponding period of time must be provided for the additional pulses.
In the MR imaging method described in European Patent 0 752 596 additional gradient pulses are added to the gradient pulses required for the imaging in order to return the residual magnetization to zero. Multi-echo sequences, however, are not described therein.
An object of the invention is to provide a multi-echo imaging method by which artifacts as a result of residual magnetizations are reduced without extending the measuring time.
This object is achieved in a multi-echo MR imaging method wherein at least two compensation phase encoding gradient pulses are generated between the high-frequency exciting pulse and the first high-frequency refocusing pulse, to counteract the influence of the residual magnetization as a noise field on the echo paths, so that signal deletions as a result of destructive interferences no longer occur at least in the echo paths of high signal intensity. The properties of the sequence do not worsen, for example with regard to the repetition time, echo time, number of slices etc., when gradient pulses are added between the high-frequency exciting pulse and the first high-frequency refocusing pulse. It is not necessary to know the disturbance variable for calculating the compensation phase encoding gradient pulses; the variable of the disturbance, therefore, must not be determined prior to the actual measuring.
The amplitudes of the compensation phase encoding gradient pulses can be simply determined when they correspond to the amplitude of the following phase encoding gradient pulse.
In another embodiment of the invention the chronological distance of the compensation phase encoding gradient pulses from one another is half of the chronological distance of the following two phase encoding gradient pulses from one another. Thus the phase error generated between the high-frequency excitation pulse and the first high-frequency refocusing pulse is half as large as the maximum phase error. The maximum phase error occurs between the first high-frequency refocusing pulse and the second high-frequency refocusing pulse.
As is subsequently explained in greater detail, the differences regarding the phase errors of the echo paths, which are crucial for the image quality, are adjusted to a minimum, so that the disturbing signal interferences are significantly reduced. Regardless of the remanence effects, there are similar influences on the image quality by the Maxwell terms (quadratic gradient terms). In order to also avoid disturbances resulting therefrom, the pulse duration of the compensation phase encoding gradient pulses, in a further embodiment, is half as long as the pulse duration of the two following phase encoding gradient pulses.
In another embodiment of the invention, the pulse durations of the phase encoding gradient pulses are varied between the individual high-frequency refocusing pulses, with the pulse amplitudes being essentially of the same magnitude. In particular, highly remanent noise fields are prevented. The pulse durations and the pulse amplitude are selected such that the time integrals of the gradient pulses are unmodified relative to the non-compensated original sequence.
In a further embodiment of the invention the phase encoding gradient pulses, between the high-frequency pulses, have a time integral value corresponding to the sign-inverted time integral value of noise fields between the corresponding high-frequency pulses. The variable of the noise fields must be known or determined in this specific embodiment; the compensation, however, can be almost ideally carried out.